Compositions comprising an epoxy resin and a curing agent have been known for decades. Many curing agents are reactive with the epoxy resin at room temperature and, therefore, need to be mixed with an epoxy resin immediately prior to use. These compositions are known as two component (2K) epoxy systems. Others known as latent hardeners are stable in an admixture with the epoxy resin at ambient temperature and effect hardening only when heated to elevated temperature. Some compounds also act as accelerators of the latent curing agent, dicyandiamide (DICY) and effect cure of epoxy resins at elevated temperatures. These are known as one component (1K) epoxy curing systems. In some cases, a one-component epoxy based adhesive system is preferred over a two-components system because it eliminates the mixing step, the required time to apply it, and the cooling during storage and shipping associated with the two-component system.
Most conventional one component (1K) epoxy curing systems are suspensions of solid compounds or mixtures in an epoxy resin which are latent by virtue of their low solubility in the epoxy matrix. Examples of these are described in U.S. Pat. Nos. 3,519,576; 3,520,905; and U.S. Pat. No. 4,866,133 in which the curing agents are solids formed from polyamines with phenols. The polyamines contain at least two amine groups with at least one being a primary amine.
In U.S. Pat. No. 4,689,390 a latent curing agent was prepared by reacting a diamine bearing a tertiary amine and a primary or secondary amino group with a poly-epoxy compound and a phenolic resin or phenolic compounds. A solution of a polyamine adduct with bisphenolic A diglycidyl ether in poly-phenolic resins was described in US Patent Pub No US-2012-0077943-A1. U.S. Pat. No. 7,910,667 describes a polyphenolic resin solution of a polyurea derivative of a polyamine which has been used as a latent epoxy curing agent.
WO 2008/152004 A1 discloses using liquid imidazolium salts (ionic liquids) for curing epoxy compositions. These liquids are described as latent catalysts for these epoxy compositions. U.S. Pat. No. 3,839,281 describes the use of N-hydroxyethyl piperidines and piperazyl compounds as accelerators for epoxy resins systems cured with DICY.
The previously identified patents and patent applications are hereby incorporated by reference.
Liquid one component (1K) epoxy curing agents and DICY accelerators offer advantages over solid curing agents. Liquids can provide better mixing than solids which is valued in certain applications. For example in applications involving electronic components, liquid curing agents reduce the potential for “hot spots” within the curing epoxy resin which can damage these components. As a result, there is a need in this art for liquid latent epoxy curing agents which exhibit prolonged storage stability at ambient temperature and cure rapidly at temperatures greater than about 80° C.